Viscoelastic and fragmentation characters of model bolus from polysaccharide gels after instrumental mastication

Model bolus from polysaccharide gels was investigated by the stress-relaxation tests and particulate size analyses. Using two gelling agents, gellan gum and a composite of gellan/psyllium seed gums, gels with different physical properties (i.e., elastic gellan single gels and plastic composite gels) and gel hardness were prepared. Gels were masticated instrumentally in the presence or absence of artificial saliva to prepare model bolus. Data from the stress-relaxation tests analyzed by 5-element mechanical model showed that difference between two Maxwell-bodies in the elasticity for the composite gels was generally smaller than that for gellan single gels when compared at equivalent gel hardness and was less influenced by the addition level of saliva. For each gel sample, cumulative particulate size distribution of model bolus was reduced logarithmically with a normal curve regardless of the addition level of saliva. Mean particulate size of model bolus from the composite gels was generally larger than that for gellan single gels when compared at equivalent gel hardness and was less influenced by the addition level of saliva. Based on the particulate size distribution of model bolus, coefficients of skewness and kurtosis calculated for the composite gels tended to be larger than those for gellan single gels when compared at equivalent gel hardness. Results indicated higher structural homogeneity of model bolus from the composite gels, which is related to higher miscibility with saliva. Structural homogeneity should be the key for texture design of nursing-care foods, particularly for dysphagia.     

Aeration of model gels: Rheological characteristics of gellan and agar gels

Model food sols containing gellan and agar at different solid concentrations were aerated. The rheological characteristics of the setted gels were determined. These were the compression characteristics (fracture strain/stress/energy, firmness and the total energy for compression) and the stress relaxation characteristics (extent of relaxation and relaxation time). Fracture strain of gels increased due to aeration indicating that the aerated samples exhibited a delayed fracture due to the presence of incorporated air. The extent of increase in fracture strain for aerated agar gels over the corresponding non-aerated samples is 1.7-23.1% while it is 4.3-15.1% for gellan gels. The extent of relaxation increased due to aeration while relaxation time (λ) decreased. Relaxation time for aerated agar and gellan gels is 9.6-40.7% and 19.6-37.6% lower than the non-aerated samples. At a low concentration of hydrocolloid, the effect of aeration was usually non-significant (at p ? 0.05) while prominent changes in rheological behaviour occurred at a high concentration. Aeration of food gels offered the possibility of rheological modifications to suit the consumer requirements in developing specialty fabricated gels.     

Stress relaxation behaviour of high acyl gellan gels

Stress relaxation behaviour of high acyl gellan gels has been investigated, and data were fitted successfully by a seven elements empirical model and a four term modified Maxwell model with three fixed relaxation times (20, 200 and 2000 s). In addition, the effect of testing parameters on stress relaxation characteristics and the relationship between those characteristics and intrinsic gel properties were studied. High acyl gellan gels were tested in stress relaxation with different cross‐head speeds (0.1–10 mm s^?1) to applied strains (3–30%). The results showed that the cross‐head speed had little effect on stress relaxation behaviour of gels. With increasing the applied strain, the initial stress and the equilibrium stress increased. The equilibrium stresses from relaxation measurement were positively related to the hardness from TPA tests (R² = 0.991). Relaxation appeared to be associated with the shifting of cross‐links in the gel matrix.     

CHARACTERIZATION OF GELLAN GELS BY UNIAXIAL COMPRESSION,STRESS RELAXATION AND CREEP 1

The rheological properties of 0.5–2.5% gellan gels were evaluated by uniaxial compression, stress relaxation and creep tests. The gel's strength was in the range of 0.1–1 kg.cm^-2 and their deformability modulus 1–6 kg.cm^-2. The asymptotic modulus determined in relaxation tests at two strains and the asymptotic creep compliance determined under two loads indicated that gellan gels have a yielding structure. The strains sustained in creep were considerably higher than the failure strain in uniaxial compression. Weight loss due to syneresis was on the order of 4–38% depending on the gum concentration, the deformation level in relaxation or the load in the creep tests.     

Comparison of Gelation Mechanism of Surimi between Heat and Pressure Treatment by Using Rheological and NMR Relaxation Measurements

ABSTRACT: The stress-relaxation and nuclear magnetic resonance (NMR) proton spin-spin relaxation time (¹H T₂) measurements were performed to elucidate the gelation mechanism of heated and pressurized surimi gels. Numerical inverse Laplace transformation of the stress-relaxation data showedlarge distributions of stress-relaxation times and the Kohlrauch-Williams-Watts (KWW) equation described well the stress-relaxation process for each gel. NMR measurement gave information about the content of the exposed protein surface. From these results it is considered that the heat treatment induces the denaturation and re-aggregation of protein, which contribute to the formation of strong network structure. On the other hand, the pressure treatment contributes to the formation of weak cross-links made by the aggregation of protein in native form.     

Compressive textural attributes, opacity and syneresis of gels prepared from gellan, agar and their mixtures

Gellan, agar and their mixed gels at an equal proportion of both were subjected to uniaxial compression to determine the textural attributes including fracture characteristics, opacity and syneresis. An increase in concentration of hydrocolloids increased opacity but decreased syneresis. Highly transparent gellan gels with opacity less than 8% were obtained; opacity of agar gels was highest, and was between 5% and 30% for their mixed gels. Syneresis as low as 1.5% was observed with the mixed gel containing 2% each of agar and gellan. The maximum stress and energy for compression increased with the level of gelling agents while instantaneous relaxation in height decreased. Binary combinations or mixed gels containing agar and gellan can provide unique textural and physical characteristics like syneresis and opacity.     

结冷胶-羧甲基纤维素复配胶性能的研究

利用全质构分析、离心分离和分光光度法研究了结冷胶-羧甲基纤维素（carboxyl methyl cellulose,CMC）的配比、离子种类和离子浓度对结冷胶-CMC复配胶质构、持水力以及透明度的影响。结果表明：复配胶硬度随结冷胶比例降低而降低;在离子浓度达到临界水平前随离子浓度增加而增加,临界水平之后随离子浓度增加而降低;添加钙离子比钠离子更容易得到高硬度的凝胶。总的来说,添加离子会降低凝胶的弹性和内聚性;适当的结冷胶和CMC比例可提高凝胶弹性和内聚性。降低结冷胶-CMC复配比例可提高复配胶的透明度。     

Serum release boosts sweetness intensity in gels

This paper describes the effect of serum release on sweetness intensity in mixed whey protein isolate/gellan gum gels. The impact of gellan gum and sugar concentration on microstructure, permeability, serum release and large deformation properties of the gels was determined. With increasing gellan gum concentration the size of the pores present in the protein network, the permeability and the serum release increased, as well as the Young's modulus, the fracture stress and the fracture strain. Increasing the sugar concentration induced an increase of the pore size, but resulted in a decrease of permeability and serum release. The addition of sugar resulted in gels with a higher Young's modulus and a lower fracture strain. This effect was more evident at higher gellan gum concentrations. By changing the protein concentration of the gels, a set of samples was prepared exhibiting constant large deformation properties but varying in serum release and sugar concentration. Serum release significantly boosted sweetness intensity. For example, the sweetness scores for gels with 12% serum release were the same as for gels with 2% serum release but 30% higher sugar concentration. The results indicate that serum release is a tool to compensate for the loss taste intensity related to the reduction of sugar and salt in gelled foods.     

Comparative assessment of textural properties and microstructure of composite gels prepared from gelatine or gellan with maize starch and/or egg white

Single component gels (SCG) were formed from gelatine, gellan, maize starch or egg white, while binary composite gel (BCG) and ternary composite gel (TCG) were formed by mixing gelatine or gellan with maize starch and/or egg white. Each type of SCG exhibited distinct textural and microstructure characteristics. Gelatine‐SCG was the softest to hold but has the strongest and the most elastic network compared with other SCG. The effects of gelatine or gellan proportions on the textural properties of composite gels were investigated using mixture design experiments. Gelatine and gellan yielded composite gels that showed different textural behaviours and microstructure. All BCG and TCG blends showed antagonistic effects; the composite gels were softer and weaker as compared to the SCG. Gelatine‐BCG and TCG were comparatively stronger than those of gellan due to their different gelling mechanisms, in which the former had yielded a denser network structures as compared to the latter.     

Texture profile and turbidity of gellan/gelatin mixed gels

The effect of gellan (1.6–0.2%) to gelatin (0–1.4%) ratio and calcium ion concentration (0–30 mM) on the textural properties and turbidity of gellan/gelatin mixed gels was examined using instrumental Texture Profile Analysis (TPA) and spectrophotometry. Hardness of the mixed gels decreased as the proportion of gellan decreased. Hardness increased with increasing calcium ions until calcium concentration reached a critical level, after which further increases in calcium resulted in a reduction of hardness. Brittleness, springiness and cohesiveness were very sensitive to low levels of added calcium (0–10 mM), but less sensitive to higher calcium concentrations and gellan/gelatin ratio. In general, the addition of calcium ions caused gels to be more brittle and less cohesive and springy. Decreasing gellan to gelatin ratio caused an increase in gel turbidity at lower calcium ion levels (2–4 mM) and a decrease in turbidity at high calcium levels (20–30 mM). Maximum turbidity was observed in 0.6% gellan–1.0% gelatin gels without added calcium. The results of this study suggested a weak positive interaction between gellan and gelatin when no calcium was added, whereas at higher calcium levels gellan formed a continuous network and gelatin a discontinuous phase.     

Swallowing profiles of food polysaccharide gels in relation to bolus rheology

Swallowing profiles of food polysaccharide gels were investigated in relation to bolus rheology. Polysaccharide gel from either gellan gum or a mixture of gellan gum and psyllium seed gum was used as a model food. Acoustic analysis and sensory evaluation were carried out to investigate the swallowing profiles using the same human subjects. Model bolus was prepared through instrumental mastication using a mechanical simulator to mimic the action of the human jaw in the presence or absence of artificial saliva and was subjected to dynamic viscoelasticity measurements to investigate the rheological properties. Bolus from the binary gel was shorter in time required to transfer through the pharyngeal phase due to mass flow and was scored higher in sensory perceived cohesiveness (bolus forming) than that from gellan gum gel. Model bolus from the binary gel showed a rheologically weak gel (or structured fluid) behavior and was higher in structural homogeneity than that from gellan gum gel. Also, dynamic viscoelasticity parameters of the binary gel were less dependent on the addition level of saliva. Results indicate that the viscoelasticity balance is a key for texture design of dysphagia foods in relation to the saliva miscibility.     

低酰基结冷胶酸性凝胶的凝胶特性

以葡萄糖酸-δ-内酯(glucono-δ-lactone,GDL)作为酸诱导剂,制备低酰基结冷胶(low acyl gellan gum,LA)酸性凝胶,考察基体质量浓度、GDL/LA复配比例以及酸液浸泡对酸性凝胶凝胶特性的影响。研究结果表明,GDL酸化为缓慢酸化,GDL/LA复配比例越高、体系的pH值越低,酸化速率越快。基体质量浓度和GDL/LA复配比例对酸性凝胶结构影响显著,断裂应力和保水性随着GDL/LA复配比例的增大先升高后降低。基体质量浓度越高,断裂应力和不透明性越大。GDL/LA复配比例增大,断裂应变减小,不透明性增大。当酸液pH值为1时,酸液浸泡对GDL/LA复配比例为2∶1和4∶1的酸性凝胶强度无影响,但GDL/LA复配比例为1∶4、1∶2和1∶1时,凝胶强度随浸泡时间的增加而增强,酸液浸泡可以促使酸性凝胶进行结构重建。     

Characterizations of fish gelatin films added with gellan and κ-carrageenan

Fish gelatin is known to be inferior to mammalian gelatins. Gellan and κ-carrageenan were added to improve properties of the fish gelatin films. Initially, polysaccharides were added to make fish gelatin gels, and tested for the melting point. Mechanical, barrier, color and microstructure properties, as well as Fourier transform infrared (FTIR) and thermal analysis (DSC) of the modified fish gelatin films were evaluated. The addition of gellan and κ-carrageenan increased the melting point of fish gelatin gels, gellan being more effective. Polysaccharides modified fish gelatin films by increasing tensile strength and barrier against water vapor, but made films slightly darker. Scanning electron microscopy (SEM) microstructure analysis revealed that gellan eliminated cracks present in the film matrix resulting in a more uniform structure. FTIR and DSC analyses showed that both polysaccharides effectively interacted with fish gelatin, and moreover, gellan being more effective. Overall, addition of gellan up to 2 g/100 g of gelatin performed better in enhancing fish gelatin films properties.     

RECOVERABLE WORK VERSUS ASYMPTOTIC RELAXATION MODULUS IN AGAR,CARRAGEENAN AND GELLAN GELS1

Cylindrical specimens of agar, carrageenan and gellan gels (1&2%) were uniaxially compressed to various predetermined strain levels and then decompressed. In an independent set of tests, specimens of the same gels were compressed to the same strains and then allowed to relax. Both the percent recoverable work, calculated from the stress-strain relationships in the compression-decompression cycles and the magnitude of the asymptotic relaxation modulus, calculated from the normalized and linearized relaxation curves, decreased as the strain increased. In all three gels there was a linear correlation between the percent recoverable work and the magnitude of the asymptotic modulus at corresponding strains, a sign that both parameters are indicative of these gels’ degree of elasticity and the latter's strain dependency.     

EFECT OF pH BUFFERS ON MECHANICAL PROPERTIES OF GELLAN GELS

The strength and deformability of calcium cross-linked gellan gels as affected by pH 3.5 and 5.0 citrate and acetate buffers were measured by large compressive deformation test until failure. The trend of dependence of gel strength on polymer and calcium concentrations was similar to gels formed in distilled water without pH adjustment. A critical calcium concentration was observed for each gellan concentration. Gels formed at the critical calcium concentration exhibited the maximum strength. The chelating effect of pH 5.0 citrate buffer greatly increased the critical calcium concentration. The failure strain, representing the deformability, of gellan gels formed in buffers behaved differently from gels formed in distilled water. In the pH 3.5 buffer systems, gellan gels were brittle regardless of gellan and calcium concentrations. In the pH 5.0 buffer systems, gellan gels were brittle at high calcium concentrations and ductile at calcium concentrations less than 24 mM in citric buffer and less than 6 mM in acetate buffer.     

Effect of substitution of aspartame for sucrose on instrumental texture profile of hydrocolloids gelled systems

The effect of substituting aspartame (0.04–0.16% w/w) for sucrose (5–25% w/w) on the instrumental texture profile of three hydrocolloid gelled systems: -carrageenan, gellan gum and -carrageenan/locust bean gum at three different concentrations (0.3, 0.75 and 1.2% w/w) was studied. In gellan gels hardness and chewiness increased with sucrose concentration but not so in either -carrageenan or -carrageenan/LBG gels where no changes in these parameters were detected. Addition of sucrose also produced a small but significant increase in cohesiveness in gellan gels. The effect of sucrose concentration on all texture parameters of gellan gels was higher as gellan concentration increased. Aspartame addition did not affect TPA parameters. TPA results obtained for aspartame gels did not differ from those corresponding to the unsweetened gels considered here. Summarising the above information it can be said that substituting aspartame for sucrose in this type of sweetened gel changed the textural properties and that the changes produced depended on the hydrocolloid type and concentration and on the sucrose concentration replaced.     

TEXTURE PROPERTIES OF GELLAN GELS AS AFFECTED BY TEMPERATURE

Gellan gels can be made very brittle, similar to agar gels, or very flexible, like gelatin gels. The entropy or enthalpy nature governing those gellan gel behaviors was studied by mechanical testing at temperatures varying from 2 to 62C. Both failure stress and strain for 1% low acyl and low acyl/high acyl mixed gellan gels decreased with increasing temperature, indicating that the hydrogen bonding contributed significantly to the stabilization of gellan gels in addition to the polyanion-calcium-polyanion bonding. Hydrophobic interactions were less important. The initial Young's modulus for two mixed high and low acyl gellan gels containing 2 mM Ca⁺⁺ increased with temperature from 2–42C, indicating entropy elasticity. Average molecular weight between adjacent crosslinks for these two mixed gels was larger than 10⁴. For other gels, the entropy elasticity was not a dominant mechanism for elastic force because of molecular weights between crosslinks and from the observation of negative temperature dependence of the modulus.     

Gellan gum/cassava starch mixtures in water systems and in milk systems

Cassava starch is an important ingredient in various foods. However, when processed, it develops some properties that are unsuitable for many industrial applications, such as unstable viscosity and excessive cohesiveness. To reduce those disadvantages, one alternative is a mixture of starch with hydrocolloids. The aim of this study was to evaluate the effect of 0:05, 0.1, 0.2, or 0.3% w/v gellan gum on the physical properties of pastes and gels of cassava starch in water and in milk systems. Differential scanning calorimetry (DSC), pasting properties, the texture profile, and the microstructure of gels were studied. The addition of gellan gum dispersions to the water system (WS) and the milk system (MS) gave rise to pastes with higher viscosity and, in the case of MS, with lower thermal and shear stability (higher breakdown). The addition of gellan gum had a greater textural effect on the MS, in which the addition of gellan at concentrations of 0.1 and 0.3% increased adhesiveness and decreased springiness of gels. Microscopy revealed a more uniform gel structure in the MS compared with the WS. Starch gelatinization temperatures were higher in MS than in WS.     

Rheological analysis of emulsion-filled gels based on high acyl gellan gum

Emulsion-filled gels are widely used in cosmetic, food, and pharmaceutical industry. As rheological properties of these systems are strongly dependent on the properties of the gelled polymer network, rheological characteristics of gels containing high and low acyl gellan gum were analyzed. Under the processing conditions low acyl emulsions were unstable, thus in the present work the influence of oil and hydrocolloid concentrations on the viscoelastic behavior of emulsion-filled gels containing high acyl gellan gum was studied. Increasing gellan concentration (from 0.1 g/100 g to 0.5 g/100 g) produced stronger gels, while oil fraction (10 g/100 g–30 g/100 g) slightly affected the elastic behavior of the emulsions reinforcing the structure and the elastic characteristics of the gellan matrix. Sauter diameter (d₃₂) was measured for all emulsions and an average value of 12 μm was obtained. Rheological data (oscillatory and creep–recovery tests) were successfully modeled to interpret the structural characteristics of the gelled emulsions. The broadened Baumgaertel–Schausberger–Winter spectrum was used to represent the linear viscoelastic behavior of the continuous phase and the emulsified system, showing that the rheological behavior of the systems was controlled by the highly structured continuous phase rather than the contribution of filler lipid droplet in the emulsion. Relaxation spectra were validated using creep–recovery experiments. Regardless of hydrocolloid concentration, creep compliance of the gel emulsions decreased compared with their respective gels, showing that the inclusion of oil droplets produced a reinforcement of the structure and the gel strength of the matrix.     

Influence of equibiaxial extensional strain on stress relaxation of glycerol plasticised wheat gluten

BACKGROUND: Glycerol plasticised wheat gluten shows advantages for non‐food applications such as biodegradable package films and bioplastics because of their abundant resources, low cost, good biodegradability and suitable mechanical and oxygen barrier properties. The intention of this study was to evaluate to the equibiaxial stress relaxation of 400 g kg^?1 glycerol plasticised gluten at different biaxial strains at room temperature for a better understanding of the processibility. RESULTS: The plasticised gluten shows significant stress relaxation and the spectra span over six decades of time at biaxial strains from 0.03 to 1.51. Plotting the instantaneous modulus against strain reveals strain softening and strain hardening at strains below and above 0.25, respectively. The relaxation modulus as a function of time can be fitted to the Kohlrausch–Williams–Watts equation. CONCLUSION: These results suggest that stress relaxation of plasticised gluten is highly dependent on strain level and biaxial deformation accelerates the network relaxation by widening the distribution of relaxation times. Copyright © 2008 Society of Chemical Industry